Certain steam generators, and in particular those used in nuclear power stations, have 3000 or more U-shaped tubes, whose branches have a length of about ten meters. These tubes are introduced into a series of perforated spacer plates and their ends are expanded into a tubular plate in contact with the primary water box.
The openings permit an arrangement of the tubes in the form of planar, parallel layers within which they are concentrically positioned in accordance with the spacing of their branches and the radius of the U-shaped part. Moreover, the width of the layers decreases with their distance from the central layer and, as the various layers are centered on the same plane, the general contour of the zone of the U-shaped parts is that of a hemisphere. As the layers are very compact, by analogy of appearance this is called a chignon zone.
The problems which occur in this type of installation are due to vibratory phenomena occurring in the steam generator. The latter has a very elongated shape and is sensitive to the oscillations which are more marked at its top where the chignon is located. The impossibility of arranging supplementary spacer plates at this location, associated with the flexibility of the tubes and their different free lengths, produces complex vibratory phenomena which must be eliminated.
Therefore, anti-vibratory bars are inserted between each planar layer of tubes, whose ends, which project somewhat from the chignon, are then welded to semi-circular "hairpins" disposed in accordance with meridians. Thus, a much more rigid assembly is obtained and its vibratory behavior is improved.
However, the positioning of anti-vibratory bars between closely spaced layers with a width of several meters and constituted by tubes which are to a greater or lesser extent deformed, and of which those located within the chignon are virtually inaccessible, still causes certain problems. Insertion is difficult even after the preliminary introduction of a nylon thread between two layers of tubes. Furthermore, excessively buckled or bent tubes are almost always subject to scratches, which in each case constitute corrosion starting points and which are even more likely to occur if installation requires the horizontal positioning of the generator and the lower layers are then highly compressed by the weight of the other layers. After installation, the tubes are inaccessible and their detects cannot be. It is merely possible to attempt to prove by means of long-armed probes that the clearances between the anti-vibratory bars and certain tubes are not excessive. These measurements, performed under poor conditions, are excessively imprecise and are no longer considered reliable. Therefore attempts have been made to develop processes using eddy current, which can at the best establish that defects are absent, but cannot obviate them.
This state of affairs has led to mistrust on the part of the organizations responsible for examining the safety of nuclear power stations, while it is impossible to bring the layers of tubes as close together as might be desired.